Bentonite is typically used as the binder in a method of the type described above, such as EP 2 014 391 [U.S. Pat. No. 8,029,711] by the instant patent applicant, for example, or U.S. Pat. No. 4,505,750. The expandable additive, which is added, may be perlite, vermiculite or expandable graphite. The expandable additive has an expanding index of at least 9, i.e. the expandable additive in question multiplies its volume accordingly at a certain temperature. This temperature is typically 300° C. This prevents harmful emissions in particular and improves the quality of the casting.
Inorganic binders such as bentonite according to EP 2 014 391 have the fundamental advantage in comparison with organic binders that they release significantly fewer pollutants during casting. In addition to bentonite as an inorganic binder for molds and cores, molding material mixtures may basically be used to make metal-processing casting molds that rely on a water glass-based binder as described in DE 10 2004 042 535 [U.S. Pat. No. 7,770,629].
In general, the individual grains of the granular, mineral and refractory mold base material are bonded and/or glued to one another with the help of the inorganic binder. The mold base material is typically sand and/or quartz sand. The physical curing of the binder from water glass, for example, usually takes place by heating, namely by extracting moisture by drying. The drying may take place in a hot core box, by gassing with hot air in the respective core box or with the help of microwave heating and/or in a traditional furnace.
After curing, the grains of the mold base material are bonded together by binder bridges created with the help of the binder. The additional expandable additives added within the scope of EP 2 014 391 or according to U.S. Pat. No. 4,505,750 ensure that removal of the core is facilitated because the expandable additive ensures that the core can be separated from the casting, for example.
Problems can often occur at this point because of the special characteristics of the inorganic binder and in particular the water-glass binders, so that separation of the core and/or core sand and/or mold sand from the cast part is incomplete or only partially successfully. For example, DD 158 090 relates to a method for regulating the strength of inorganic molding materials based on alkali silicate solutions. The special characteristics of water glass as a binder are described here and the unsatisfactory disintegration properties are also presented in this context.
These disintegration properties must now be mapped in the most accurate and error-free manner possible. Surface defects, for example, may occur when particles and/or grains become detached. Furthermore, single-phase or multiphase inclusions that may be observed at the surface of the casting can be attributed to reactions of the core sand and/or mold sand with the melt. Such inclusions are sometimes macroscopic, i.e. visible to the naked eye, and usually affect the mechanical properties of a casting. In an extreme case, this may result in rejects.
In production of the cast parts, the focus not just on a satisfactory cast surface but instead the entire manufacturing process makes particularly high demands of core removal. In other words, after the casting has been produced, it is important for the molds and cores to be separated satisfactorily from the casting. To support this process mechanical energy is often applied by shaking or vibrating that, in addition to the expandable additive, ensures that the binder bridges between the individual grains are destroyed and consequently, in the ideal case, the mold base material trickles out of the casting freely.
In particular in the case of cores with inorganic binders and having narrow dimensions in the millimeter range, such as those observed in particular with cylinder heads having a water jacket for production of automotive engines, core-removal problems occur and may be further exacerbated by the relatively low casting temperatures of the aluminum alloys that are generally used here. In other words, with the thin passages that are formed, core sand residues may often remain adhering in or may even block the passages. In addition, inorganic binders based on bentonite are generally associated with the disadvantage that the casting molds produced from them have a relatively low strength. However, a high strength is especially important when making such complicated thin-walled cores (molds) and for their safe handling.
The reason for the low strength of bentonite-bonded molds and cores in comparison with molds and cores bonded with water glass, for example, can be attributed essentially to the fact that the bentonite-bonded casting molds have a slightly different binding mechanism and still contain residual water from the binder. This is where the invention begins.